Panchromatic film
Black and white film that is sensitive to all visible wavelengths.

Parallax
Displacement of the position of a target in an image caused by a shift in the observation system.

Parallax difference
The difference in the distance on overlapping vertical photographs between two points, which
represent two locations on the ground with different elevations.

Parallel-polarized
Describes a radar pulse in which the polarization of the return is the same as that of the transmission. Parallel-polarized images may be HH (horizontal transmit, horizontal return)
or VV (vertical transmit, vertical return).

Pass
In digital filters, refers to the spatial frequency of data transmitted by the filter. High-pass filters transmit high-frequency data; low-pass filters transmit low-frequency data.

Passive microwaves
Radiation in the 1 mm to 1 m range emitted naturally by all materials above absolute zero.

Passive remote sensing
Remote sensing of energy naturally reflected or radiated from the terrain.

Path-and-row index
System for locating Landsat MSS and TM images.

Pattern
Regular repetition of tonal variations on an image or photograph.

Periodic line dropout
Defect on Landsat MSS or TM images in which no data are recorded for every sixth
or sixteenth scan line, causing a black line on the image.

Periodic line striping
Defect on Landsat MSS or TM images in which every sixth or sixteenth scan line is brighter or darker than the others. Caused by the sensitivity of one detector being higher or lower than the others.

Photodetector
Device for measuring energy in the visible-light band.

Photogeology
Mapping and interpretation of geologic features from aerial photographs.

Photogrammetry
Technology used to define the shape and position of objects from a photograph, and to determine their characteristics without touching them.

Photograph
Representation of targets on film that results from the action of light on silver halide
grains in the film's emulsion.

Photographic IR
Short-wavelength portion (0.7 to 0.9 µm) of the IR band that is detectable by IR color film or IR black-and-white film.

Photographic UV
Long-wavelength portion of the UV band (0.3 to 0.4 µm) that is transmitted through the atmosphere and is detectable by film.

Photomosaic
Mosaic composed of photographs.

Photon
Minimum discrete quantity of radiant energy.

Picture element
In a digitized image, the area on the ground represented by each digital number.
Commonly contracted to pixel.

Pitch
Rotation of an aircraft about the horizontal axis normal to its longitudinal axis that causes a nose-up or nose-down attitude.

Pixel
Contraction of picture element.Acronym for picture element. Digital images in raster format consist of a two-dimensional grid. A pixel is the element at the intersection of a column and a row in the grid. Each pixel has an intensity value which can vary through various shadings of gray or colors.

Planck's Law
An expression for the variation of emittance of a blackbody at a particular temperature as a function of wavelength.

Point spread function (PSF)
The image of a point source of radiation, such as a star, collected by an imaging device. A measure of the spatial fidelity of the device.

Polarization
The direction of orientation in which the electrical field vector of electromagnetic radiation vibrates.

Polar orbit
An orbit that passes close to the poles, thereby enabling a satellite to pass over most of the surface, except the immediate vicinity of the poles themselves.

Polarized radiation
Electromagnetic radiation in which the electrical field vector is contained in a single plane, instead of having random orientation relative to the propagation vector. Most commonly refers to radar images.

Positive photograph
Photographic image in which the tomes are directly proportional to the terrain brightness.

Previsual symptom
A vegetation anomaly that is recognizable on IR film before it is visible to the naked eye or on normal color photographs. It results when stressed vegetation loses its ability to reflect photographic IR energy and its recognizable on IR color film by a decrease in brightness of the red hues.

Primary colors
A set of three colors that in various combinations will produce the full range of colors in the visible spectrum. There are two sets of primary colors, additive and subtractive.

Principal component analysis
The analysis of covariance in a multiple data set so that the data can be projected as additive combinations on to new axes, which express different kinds of correlation among the data.

Principal-component (PC) image
Digitally processed image produced by a transformation that recognizes maximum
variance in multispectral images.

Principal point
Optical center of an aerial photograph.

Printout
Display of computer data in alphanumeric format.

Probability density function (PDF)
A function indicating the relative frequency with which any measurement may be expected to occur. In remote sensing it is represented by the histogram of DN in one band for a scene.

Pulse
Short burst of electromagnetic radiation transmitted by a radar antenna.

Pulse length
Duration of a burst of energy transmitted by a radar antenna, measured in microseconds.

Pushbroom scanner
An alternate term for an along-track scanner

Pushbroom system
An imaging device consisting of a fixed linear array of many sensors which is swept across an area by the motion of the platform, thereby building up an image. It relies on sensors whose response and reading is nearly instantaneous, so that the image swathe can be segmented into pixels representing small dimensions on the ground.